In impact printers, such as daisy wheel printers, wherein formed character elements upon a print wheel are impacted and driven toward a platen by a hammer mechanism, or dot matrix printers wherein characters are formed as selected combinations of reciprocating wires are driven toward a platen, a significant determinant of output print quality is the size of the gap between the surface of the recording medium held against the platen and the impacting element. Deviations from an optimal printing gap will either cause the impression to be too light--if the distance is too great, or may emboss the recording medium--if the distance is too small.
Various methods for effecting the gap adjustment between a print head and a platen are well known in the art. In most serial printers, a print head is slidably mounted on a pair of guide rods which are positioned parallel to the platen and are held in place by having their ends fastened between the left and right side walls of a printer frame. One guide rod is positioned near the platen and passes through an opening in the front portion of the print head. The second guide rod is mounted so as to support the back portion of the print head and passes through an opening in the back portion of the print head, the opening having a shape such as to permit lateral motion between the print head and the guide rod. The guide rod passing through the front portion of the print head is mounted on a pair of eccentrics which permit the rod lateral motion towards and away from the platen. A similar print head mounting arrangement is described in U.S. Pat. No. 4,525,085.
This type of print head mounting arrangement allows for a printing gap to be easily varied to accommodate multiple ply record medium, and as long as there is no relative motion between the two side walls supporting the the two guide rods and the platen, the gap will remain constant across the length of the platen. Prevention of the relative motion between the side walls, however, requires a sturdy printer frame which constitutes a major portion of the weight of the printer. To increase the portability of the printers, as well as to reduce the cost, the weight of the printer frames may be reduced to a point where the frame may not be sturdy enough to prevent relative movement between the two side walls when the printer is placed on anything but a substantially flat surface. Once there is relative motion or skewing between the side walls, the printing gap will vary across the width of the platen. FIG. 2 of the drawing illustrates the ideal condition when a printer is located on a substantially flat surface.
FIGS. 3 and 4 of the drawing show various phases of misalignment between the platen and the print head causing variation of the printing gap therebetween in a printer which is not equipped with the device of the present invention. The structures shown in these figures are described in greater detail later.